1. Field of the Invention
The present invention relates to an ultrasonic detector for detecting airborne ultrasound and in certain aspects to an ultrasonic detector for detecting high-pressure gas leaks, to methods of surveying an area for a source of airborne ultrasound, to an ultrasonic sensor and to a detector comprising such a sensor.
2. Description of the Related Art
Many industrial operations, such as well drilling, oil production, oil refining and industrial gas production, utilise piping to move a wide variety of high-pressure fluids such as gas and liquids. The pipes move such fluids for operating and controlling industrial processes amongst other things. Frequently the fluids that are piped are potentially explosive and the piping requires careful monitoring for leaks; leaks are required to be identified quickly to enable the appropriate remedial action to be taken.
Other industrial operations involving pressurised gas or those incorporating rotating parts may also produce ultrasound. In the latter case the ultrasound is due to mechanical wear of bearings or stressed components for example. Existing methods to detect such ultrasound use sensors mechanically coupled to machinery to detect vibration through the housing. However, when assemblies have multiple stress points the number of sensors required to detect wear can become prohibitive for cost reasons.
At industrial plants most man-made noise occurs in the acoustic range, whereas a pressurised gas leak produces a sound signal which spans the acoustic and ultrasonic ranges. In particular, the sound signal produced by a gas leak extends into the lower part of the ultrasonic frequency range from about 20 kHz up to about 80 kHz. The amplitude of the sound signal is a function of the size of leak, pressure in the pipe upstream and downstream of leak point, density and temperature of the leaking gas, and the temperature of the environment the gas escapes into. Although the spectra produced by two gas leaks are never identical, the one characteristic shared by the spectra of different gas leaks is that the sound signal is of a broadband type with a gradually decreasing intensity with increasing frequency. This characteristic has made gas leaks susceptible of detection.
Gas leak detectors have been devised which measure the airborne sound pressure waves generated by the turbulent flow when a gas escapes from a high to low pressure. Typically these detectors are mounted at an industrial site in a fixed position (for example on a pole or on a wall about 3 m above ground level) in order to survey a particular part of the plant. Normally several such detectors are used around the plant to ensure all important pipes, etc. are monitored.
The sensing range of such detectors is typically in the ultrasonic frequency range (25 kHz to 80 kHz for example) in order to eliminate most acoustic noise which might otherwise trigger a false alarm. An alarm level is set at some point above peak background noise level in the sensing range measured at the position where the detector is to be installed. For example, current guidelines specify that the alarm level should be approximately 6 dB above the peak background noise level. This is presently done on an installation-by-installation basis by mapping the area of interest prior to installation or by monitoring the detector output after installation. An additional precaution against false alarms is to build a time delay into the alarm circuit; two requirements must then be met to trigger an alarm: the sound pressure level of the detected ultrasonic sound must be both of a certain magnitude and duration. In this way most short-term ultrasonic sound spikes (caused by whistles, metal tags, hammering etc.) are ignored by the detector.
A further requirement is that detectors must have a means of self-test to ensure functionality and achieve Safety Integrity Levels (SIL) appropriate to pressurised gas leaks. This is currently achieved in one of two ways.
An example of the first type of self-test is described in EP 1 522 839. An externally mounted piezoelectric transducer is used to generate at predetermined intervals and for a predetermined time either a single frequency or narrowband test signal within the frequency sensing range of the detector. A microphone receives the test signal and checks whether it falls within a given tolerance range. An alert signal is generated if it does not. The narrow cone of sensitivity in front of the microphone requires that both the microphone and piezoelectric transducer to be mounted on the outside of the enclosure of the detector. Since the piezoelectric transducer and microphone are exposed to the environment, there is a problem of environmentally forced drift (explained in greater detail below), which may trigger a false alarm.
If the drift is caused by a solid build up (ice, dust etc) on the microphone, the amplitude of the test signal as detected will be reduced. This will indicate that the detector has become blocked and may have reduced coverage which is helpful, provided that the build up is actually between the piezoelectric transducer and the microphone. However, if the detector is exposed to extreme environmental conditions such as heat, cold or humidity, the signal strength of the test signal at the microphone may be either increased or decreased outside of the acceptable signal parameters. In particular, the narrowband test signal produced by the piezoelectric transducer is more susceptible to environmental extremes of temperature than the broadband pressurised leak signal to be detected. As a result the test signal emitted by the piezoelectric transducer may fall outside the tolerance levels at point of detection, (causing fault alarms) even though the detector is still functioning well within the parameters of installation. Operator confidence can be lost as a result which is undesirable. If the tolerances for the acceptable signal source are relaxed to address this problem it is possible for the receiver transducer to drift outside of acceptable calibration points.
A further problem with this first type of self-test is that mounting the test transducer on the outside of the enclosure results in a blind spot since it must protrude into the sensor's cone of sensitivity.
A second type of self-test that is more recent utilises the naturally occurring background noise to monitor the detector functionality. In particular, the background noise is used to check whether or not the receiver transducer of the detector is still operational; if the background noise is not detected the detector generates a fault alarm. The problems with this are that the receiver transducer has to have a very low sensing threshold (otherwise the self-test will not work in quiet environments), and there is no alternative way for the operator to check functionality.
In summary, existing ultrasonic gas detectors have reliability problems as far as self-testing is concerned.
Another problem facing ultrasonic gas leak detectors is variation of air temperature and humidity between the source of the gas leak and the detector. These fluctuations cause variation in the amplitude of the ultrasonic sound at the detector; in particular decreasing amplitude with increasing temperature. Therefore it is important to check that such environmental changes have not caused the detector to become ‘deaf’ to gas leaks at the current alarm threshold. For example, it is possible that a gas leak which would have just triggered an alarm may fail to do so if the temperature has increased at the plant so that the sound pressure level at the detector now falls below the alarm threshold. To overcome this problem it is known either for the detector to monitor external temperature and humidity and adjust the alarm level accordingly, or to perform regular manual calibration of the detector. The latter option is more common as this does not require additional circuitry within the detector. However, this strategy does require that detectors are either returned to the factory or calibrated in situ. Both options are time consuming as the detectors are usually mounted several meters above ground level in order to achieve maximum sensing distances. The return-to-factory method involves replicating on-site conditions such as distance, temperature, humidity and mimicking a gas leak in order to investigate detector sensitivity at various points across the detection frequency range. The on-site method uses a handheld device that comprises a narrowband signal source. The device is placed over the sensor to seal it from the environment and which places the narrowband signal source at a known distance from the sensor. By re-tuning the narrowband source to emit at different centre frequencies the broadband frequency range of interest can be investigated. One problem with both of these methods is that site personnel are required to access the detectors, and on-site calibration can only be carried out during routine maintenance schedule.
It would be beneficial in at least some embodiments of detector to have a calibration system that can recreate factory conditions and check points across the frequency range without the need to access the detector whilst it is in use on site.
Existing forms of fixed ultrasonic gas leak detector utilise a single sensing head due either to the cost of the sensor or the circuitry involved to achieve the necessary certification for use in hazardous areas. Since there is no direct interaction between leaking gas and the detector it is currently not possible to determine which type of gas is leaking. Furthermore due to the small amount of energy and the wavelengths within the sensing range, together with the nature of sound propagation through air, it is extremely difficult to ascertain the direction of the leak source using current detector design.
The amplitude of airborne sound at a given frequency at the sensor is dependant upon inter alia leak size, pressure, gas type, gas temperature and most importantly distance between leak source and sensor. Presently installation of fixed ultrasonic detectors usually provides a large coverage area which may contain multiple leak points from different pressure processes. One of the current strategies in an alarm situation is to shut down all processes within the coverage area. This is clearly costly for the plant operator and may not be necessary if the location of the leak and the associated process could be identified more accurately.
It would be beneficial in at least some embodiments of detector to provide an indication of a gas leak direction with respect to one or more detector.
Installation of gas leak detectors gives rise to further problems. In particular the current method involves mapping the background ultrasonic noise within the detector frequency range in installation area. The alarm level of detector is then manually adjusted to suit the on-site conditions. In particular, the alarm level is often set at approximately 6 dB higher than the background. One problem with this method is that the higher the background noise level, the smaller the detection range of the detector. This is due to the fact that a pressurised gas release will generate a sound pressure level which decreases at approximately 6 dB every time the distance from the source is doubled. For example, if a leak produces 102 dB at a distance of 1 m the sound pressure level will be 96 dB at 2 m, 90 dB at 4 m etc. As alarm level is usually set at 6 dB above background then the alarm level setting for an 84 dB background would be 90 dB. This will give a detection radius of 4 m for the leak example given. If background level is 90 dB current philosophy dictates that the detection radius will be 2 m. If the detection radius is too small other forms of gas detection are usually used instead, such as infra-red point detectors for their ability to identify the gas detected.
A further problem has been identified with this methodology: the background noise environment is dynamic. Therefore maximum safety alarm levels should be reviewed regularly to ensure that the sensing distance of the detector has not deteriorated.
Most ultrasonic background noise is characterised as a spike of narrowband noise (typically 1 to 2 kHz) somewhere within the detector sensing range. Such spikes are often caused by faulty mechanical components in machinery or attenuation from audible alarm devices. Due to the nature of the mechanical noise it is possible for the spikes to change frequency as the wear continues resulting in frequency drift up or down from the initial spike frequency. Such spikes can result in false alarms using existing detector functionality. The current installation method usually deals with such spikes either by reducing the detection radius of the ultrasonic detector or by selecting another detection method.
It would therefore be beneficial to provide a detector that is able to be used in areas containing electronic transducers and/or mechanical equipment and which does not trigger false alarms from ultrasonic signals emitted from the transducers and/or equipment.
To the best of the applicant's knowledge and belief all currently available sensors for detection of airborne ultrasound are of two main types: microphone and piezoelectric; each uses a different method to measure the sound pressure level.
Microphones typically use a diaphragm which moves within an electro-magnetic field or acts as one plate of a capacitor, which in turn, creates a variance in an electrical current. The electrical current can then be processed to determine the frequency and sound pressure level present at the diaphragm.
Due to the nature of construction of microphones the electro-magnetic field or area between the capacitance plates needs to be kept free from moisture and other contaminants to maintain accurate signal tolerances. When used in harsh industrial environments (e.g. on an oil drilling rig) the sensing face must be shielded from the ingress of moisture and contaminants which means they can only be mounted facing downwards which severely restricts the sensing range. A further disadvantage arises due to the use of different materials in the diaphragm and the components of the electro-magnetic field or components in the capacitor assembly and air gap. At extreme temperatures the expansion and contraction rates of said different materials results in signal drift. Microphone construction is also extremely complicated as the tolerances of the assembly need to be carefully controlled and construction must take place in a clean environment so that moisture and contaminants are eliminated during assembly, this leads to very expensive assembly costs.
Piezoelectric sensors have the ability to produce an electrical potential which is proportional to the application of mechanical pressure. Such sensors have been used in detectors, and an example of a detector of airborne ultrasound employing a piezoelectric sensor is sold by Groveley Detection Limited under the product code GDU-01. This detector comprises a piezoelectric sensor on which is mounted a damping block, or matching material as it is often known. The purpose of the damping block is to damp the natural resonance frequency of the piezoelectric so that it has a wider frequency bandwidth of sensitivity; to that end the damping block comprises a material having an acoustic impedance which is close to or that matches that of the piezoelectric.
Detectors of airborne ultrasound are often required to remain operable over a wide range of temperatures so that they can be used in harsh environments, for example on oil rigs. The GDU-01 operates satisfactorily over a temperature range of about −25° C. to +65° C. It is still workable just outside this range, although the piezoelectric crystal begins to suffer a temperature induced drift. Oil rigs are now becoming operational in places where the temperature regularly falls below or above the aforementioned range and there is a demand in the industry for detectors of airborne ultrasound which can operate reliably over a wider range of temperatures, for example between about −55° C. and +85° C.
We have identified the problem that causes the temperature induced drift: the piezoelectric crystal and the damping block each have a different coefficient of thermal expansion. At the aforementioned extremes of temperature, the relative contraction or expansion of each part has an undesired effect on the frequency response of the detector. Due to the relatively small signal (typically mV) produced by the piezoelectric crystal, the damping block must be carefully mounted to the crystal to avoid losses and therefore reduction in output. Current mounting methods include bonding and application of a viscous membrane under clamping pressure. Therefore we have recognised that whilst a piezoelectric crystal is extremely reliable in harsh environments because it is unaffected by moisture and other contaminants, it can suffer from drift at extreme temperatures due to the expansion and contraction differentials of the piezoelectric material and damping block material.
It would be beneficial in at least some embodiments to provide a detector that offers less temperature-induced drift and/or increased linearity across its detection range over a wider range of temperature, for example between about −55° C. and +85° C. The detection range may be between about 25 kHz and 80 kHz, and between about 58 dB to 106 dB.